1. Field of the Invention
This invention relates to explosive forming and in particular to the manufacture of spherical or part-spherical objects by explosive forming. The present invention may be used, in a preferred embodiment, for the manufacture of ball valve internal spheres and outer casings.
2. Description of the Prior Art
In the various engineering fields where pipelines are used to transport material, the use of valves to control or shut off the flow is common.
One type of valve, which is commonly used is a ball valve which functions by allowing the material being transported to flow through a bore in its internal sphere or ball, when the internal sphere is rotated the flow of material is cut off or reduced. This internal sphere is the most important part of the valve as it has to be accurate with regard to spherical shape and surface finish, in order to function normally.
In view of this requirement all internal spheres are manufactured either as a casting, or as a drop forging, which is then subsequently machined to the close tolerances required. As there is also a requirement for pressure and structural ability, the castings are thick and thus large and heavy. This also applies to the outer casing of the ball valve.
One specific disadvantage of the existing system is that, should any casting or forging defect exist within the thickness of the sphere, this only becomes evident during the machining operation. Often the machining operation can be in its final stages when the defect becomes apparent, resulting in the scrapping of the sphere and the loss of the machining and labour time involved. The loss factor is estimated to be high, that is, approximately 15% of production. In addition, due to the weight of the casting or drop forging, the weight of the total valve assembly is quite high, this requiring additional support systems to the structural steelwork carrying the pipeline and valve assemblies.